Atmospheric Brown Clouds:
From Local Air Pollution
to Climate Change
Guenter Engling1 and András Gelencsér2
1811-5209/10/0006-0223$2.50 DOI: 10.2113/gselements.6.4.223
A
tmospheric brown clouds are atmospheric accumulations of carbonaceous aerosol particles spanning vast areas of the globe. They have
recently gained much attention, from the scientific community and
from the general population, as they severely impact several aspects of
everyday life. Aside from affecting regional air quality and negatively
impacting human health, these clouds affect biogeochemical cycles and
profoundly influence the radiation budget of the Earth, resulting in severe
climatic and economic consequences. Carbonaceous aerosol particles are
generated primarily by combustion processes, including biomass and fossil
fuel burning. Natural emissions and transformations of volatile organic
species in the atmosphere also contribute to the development of atmospheric
brown clouds.
fast growing and poorly controlled
vehicle fleets in rapidly expanding
urban areas.
Although ABCs are found around
the globe, they are most prevalent
in Asia, Africa, and South America.
Some of the most extensive ABCs
have been observed over southern
Asia and the northern Indian
Ocean (Fig. 1). These ABCs persist
for long periods (up to 7 months
per year) and affect very large
populations. However, the haze
layer that develops annually over
Indonesia is of shorter duration
(2–3 months) but has been more
K eywords: carbonaceous aerosol, organic carbon, haze, biomass burning,
intense in certain years (a more
­radiative forcing, absorption, atmospheric brown clouds
detailed description of the
Indonesian haze is given below).
INTRODUCTION
The region in southeastern Asia
comprised
of
Thailand,
Vietnam, Laos, and Burma also
The widely used term atmospheric brown clouds (ABCs) was
experiences extensive haze development, particularly
first suggested by Ramanathan and Crutzen (2003) for
during spring when large amounts of agricultural residues
continent-sized aerosol pollution plumes. The name was
inspired by the infamous “Denver Brown Cloud,” the pollu- are burned on the fields in preparation for the new growing
tion haze hanging over the Denver metropolitan area that season. The smoke from these fires can be transported
gained notoriety in the mid 1970s. ABCs have been identi- across the Pacific Ocean as far as North America (Hadley
fied over major regional hotspots, such as the Indo-Gangetic et al. 2007). Likewise, a regional haze with a complex
chemical composition forms in eastern China, affecting
plains in southern Asia; eastern China; most of southeast
Asia, including Indonesia; regions of sub-Saharan Africa, vast metropolitan areas (including megacities), large industrial areas, and adjacent agricultural land where postMexico, and Central America; and most of Brazil and Peru.
harvest burning occurs (Lelieveld et al. 2001). ABCs in
Contrary to what the name suggests, these are not classic
South America are primarily due to smoke emissions from
bright water clouds but rather a huge blanket or layer of
the burning of rain forest (for land clearing) and agricul“haze” generally composed of light-absorbing submicrotural residues (for example, from sugar cane production)
meter-sized, carbonaceous aerosol particles. In fact, updraft
(Da Rocha et al. 2005). Slash-and-burn agricultural pracand consequently water cloud formation may even be
tices in South America entail the burning of large areas of
suppressed in ABCs due to the redistribution of solar
vegetation, including pristine rain forest, which releases
heating between the Earth’s surface and atmospheric layers
(Ackerman et al. 2000). ABCs severely affect the health substantial amounts of biomass smoke aerosol. Although
and comfort of some three billion people, and also have a such practices are often prohibited by local governments,
they are still widespread in developing countries. Even in
measurable impact on global climate, shifting monsoon
pristine areas, there may be seasonal ABCs. For instance,
patterns in southern Asia (Ramanathan et al. 2005). ABCs
a haze layer is observed each winter and early spring in
commonly originate from low-efficiency combustion
the Arctic due to long-range transport of pollution from
sources over vast and densely populated continental areas.
Such sources include residential coal burning; cooking with Europe and Russia (Quinn et al. 2009).
biofuels such as wood, dung, and crop residue; open
biomass burning associated with deforestation and cropresidue burning; and massive exhaust emissions from the
1 Research Center for Environmental Changes, Academia Sinica 128 Academia Road, Sec. 2, Taipei 115, Taiwan
E-mail: [email protected]
2 University of Pannonia, Department of Earth & Environmental
Sciences, Veszprém, Hungary
E-mail: [email protected]
E lements , V ol . 6,
pp.
223–228
THE COMPOSITION OF ATMOSPHERIC
BROWN CLOUDS
ABCs are a complex mixture of gases, vapors, and particulates. Carbonaceous aerosol particles are inherent and
major components of all ABCs, along with inorganic
species, such as sulfates, nitrates, and mineral dust. They
are largely responsible for the brown color of ABCs,
although in some cases NO2 and hematite in mineral dust
may also contribute to the color.
223
A ugus t 2010
cles (PBAPs) or bioaerosol (Heald and Spracklen 2009).
Humic-like substances (HULIS) can be another important
component of ambient aerosol particles, particularly in
remote areas or under the influence of biomass burning
(Havers et al. 1998). Biomass burning constitutes the largest
source of carbonaceous aerosols, as the practice is widespread around the world and involves the burning of
substantial amounts of plant material (biomass) for a
variety of purposes. Jimenez et al. (2009) proposed that in
situ formation of aerosol particles due to oxidation and
gas-to-particle conversion of reactive primary organic
species in the atmosphere may contribute a major fraction
of the global aerosol particle burden. In the atmosphere of
the Northern Hemisphere, the mass concentrations of these
SOA species may approach, or even exceed, the amount of
sulfate aerosol, but their chemical characteristics are still
unknown for the most part.
The terms organic carbon (OC) and elemental carbon refer to
carbon only, and the masses of these types of carbon are
measured in the laboratory using a variety of methods
(Andreae and Gelencsér 2006). The most widely used offline method is the thermo-optical determination of OC
and EC collected on quartz-fiber filters (Chow et al. 1993).
Other advances have been made recently in the compoundspecific characterization of carbonaceous aerosol particles
using techniques such as gas or liquid chromatography
coupled with mass spectrometric detection (GC-MS or
HPLC-MS). Also, the use of accelerator mass spectrometry
to determine the isotope ratio of 14C and 12C provides information about the sources of carbonaceous material. The
radiocarbon signature allows for the straightforward determination of the modern-to-fossil carbon ratio. The modern
fraction is composed of material of biological origin, such
as PBAPs, biogenic SOA, biomass-burning aerosol, and
possibly also particles from certain anthropogenic sources,
such as meat cooking. The most commonly used forms of
fossil carbon are coal, diesel, and gasoline. Recent applications of 14C analysis to the individual OC and EC fractions
have significantly improved source apportionment of
carbonaceous aerosol particles (Szidat 2009).
This Moderate Resolution Imaging Spectroradiometer
(MODIS) image, obtained from NASA’s Terra satellite,
shows the extent of an atmospheric brown cloud over northern
India and Bangladesh. It covers a land area more than 2000 km by
500 km in size, while extending hundreds of kilometers over the
Bay of Bengal. Image courtesy of Jacques D escloitres, MODIS L and
R apid R esponse Team, NASA/GSFC
Figure 1
The principal components of carbonaceous aerosol particles are organic matter (OM) and elemental carbon (EC).
Although the chemical nature of EC is not completely
known, it is rather simple in its composition compared to
OM, which consists of thousands of individual organic
compounds with a wide range of chemical and physical
properties. Many sources, both natural and anthropogenic,
contribute to the carbonaceous particle burden in the
atmosphere. Aerosol particles may be emitted into the
atmosphere directly (primary particles), but they can also
be formed from gaseous organic substances (secondary
organic aerosol, SOA). The resulting mixture has an
extremely complex composition made up of organic species
from various compound classes, such as alkanes, aromatics,
alcohols, carbonyls, carboxylic acids, and multifunctional
compounds. Certain organic constituents are toxic and
pose a risk to human health. Others are derived from the
combination of specific processes and materials and can
be used as molecular tracers for the determination of
source contributions.
Essentially all processes that burn fossil fuel or biomass
generate carbonaceous byproducts (as both gaseous and
condensed phases) in addition to the main combustion
products, CO2 and CO. Biological processes, including
microbial activity, also release significant amounts of
carbonaceous particles into the atmosphere; these particles
are typically referred to as primary biological aerosol partiE lements
TRANSFORMATION OF CARBONACEOUS
AEROSOL PARTICLES
Carbonaceous aerosol particles can be subject to physical
as well as chemical transformation processes; for example,
oxidation reactions can be induced photochemically or
through heterogeneous and multiphase chemical processes
(Gelencsér et al. 2003). The resulting SOA species lend the
particles an aged character, manifested in significantly
modified properties, such as an enhanced water-uptake
capability and a reduced tendency to volatilize (Andreae
2009). Interaction of aerosol particles with water vapor
results in changes of the aerosol constituents, both physical
(in the form of particle growth) and chemical (by inducing
aqueous-phase reactions) (Rudich et al. 2007). Consequently,
the aerosol particles may exert a fundamentally different
influence on environmental processes, such as cloud formation via the indirect aerosol effect (see later discussion).
Depending on their aerodynamic diameter, aerosol particles spend different times in the atmosphere before they
are removed by dry deposition or precipitation. Particles
with diameters ranging from 0.1 to 1 µm have lifetimes
on the order of one to two weeks and are thus subject to
long-range transport, i.e. over distances of up to several
thousand kilometers.
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A ugus t 2010
A
followed by heterogeneous reactions on the surface of other
particles, water uptake at high relative humidity, and incorporation into cloud droplets. A universal feature of BC is
that it effectively absorbs energy across the entire solar
spectrum, from the ultraviolet to the infrared. Whereas
graphite (or graphene layers, which might occur as a minor
component of diesel soot) looks perfectly black, the color
of ABCs is mostly brownish, since the absorption spectra
of forms of amorphous BC and iron-containing mineral
dust are strongly skewed towards shorter wavelengths.
B
500 nm
C
500 nm
The appearance of BC can vary significantly. For example,
diesel soot particles are typically submicrometer-sized
aggregates of individual spherules with an average diameter
of 25–35 nm (Wentzel et al. 2003). Near their emission
sources they appear as chain-like fractal structures (Fig.
2a), but atmospheric ageing gradually transforms them into
more compact grape-like clusters (Fig. 2b). Black carbon
emitted by biomass burning is characterized by char particles typically consisting of “tar balls” (Fig. 2 c) and a
mixture of unburned, partially burned, and pyrolyzed
plant materials (Fig. 2d). Coal combustion produces amorphous char particles of larger sizes consisting of mixtures
of unburned coal and fly ash spheres. Given its complexity
and variety of forms and properties, BC is by far the most
poorly quantified of all major atmospheric pollutants. The
most commonly used instrument for monitoring BC has
been the aethalometer, which is based on the continuous
measurement of light attenuation by aerosol particles
(Hansen et al. 1984). Recently, new instruments, such as
the photoacoustic spectrometer and the single-particle soot
photometer, have been developed, and these can significantly reduce the uncertainties associated with the conversion of the measured light attenuation into BC mass
concentration (Slowik et al. 2007).
D
500 nm
500 nm
Transmission electron microscope (TEM) images of carbonaceous particles from the continental troposphere. (A) A branching soot particle,
composed of primary spherules 20–50 nm in diameter; (B) an aged, compact soot
particle; (C) a “tar ball,” an amorphous, carbon-dominated particle type that is
produced by biomass burning or biofuel combustion; (D) “organic carbon” particles
with varying amounts of ammonium sulfate (the bubble-like features in the particles
are due to electron beam damage). In each image, the dark bands in the background
are from the lacey support film. Images courtesy of Mihály Pósfai
Figure 2
THE ROLE OF BLACK CARBON
IN ATMOSPHERIC BROWN CLOUDS
As inefficient combustion is a key feature of all major
sources of ABCs, an essential component is black carbon
(BC), or “soot.” Soot is virtually equivalent to elemental
carbon in aerosol, but there are distinct differences in the
definitions used by the mineralogical and atmospheric
scientific communities. Mineralogically, soot is well defined
in terms of crystallographic structure, shape, and composition: soot particles consist of aggregated spherules made
up of graphene-like layers (partially ordered carbon structures). However, in terms of its atmospheric effects, organic
matter may play a more important role than the carbon
backbone of soot particles with which it is typically associated. For instance, toxicological effects and the tendency
to adsorb water are likely more influenced by the presence
and nature of the associated OM species than the carbon
backbone of soot particles. On the other hand, in spite of
its simplified treatment in atmospheric models and environmental regulations, BC is far from being a well-defined
single material; rather, it is a continuum between highly
ordered carbon structures and complex amorphous organic
substances with markedly different physical and chemical
properties (Andreae and Gelencsér 2006). In this context,
BC can be seen as a collective term for a variety of lightabsorbing carbon species, defined operationally by
measurement methods.
Black carbon does not normally occur as discrete particles
in ABCs, but is intimately mixed with other aerosol species,
such as sulfates, nitrates, OM, mineral dust, and sea salt
(Gelencsér 2004). This high degree of mixing is attributed
to physical and chemical processes associated with atmospheric ageing. Such processes include the rapid coagulation of nanoparticles, condensation of low-volatility vapors
E lements
GLOBAL SOURCES OF BLACK CARBON
Because of analytical difficulties, mass concentrations of
atmospheric BC obtained using traditional methods must
be treated with caution, and their associated uncertainties
should never be overlooked. With this limitation in mind,
recent estimates of global BC emissions amount to 8 Mt
annually (Bond et al. 2004). This value is 1000 times less
than recent estimates of anthropogenic CO2 emissions
resulting from fossil fuel combustion, cement production,
and land-use change, which includes deforestation and
slash-and-burn practices (8000 Mt C/y; IPCC 2007). This,
however, does not mean that BC emissions can be scaled
linearly to CO2 emissions from combustion sources.
Modern power plants and industrial facilities emit very
small amounts of BC, whereas uncontrolled and inefficient
residential biofuel and coal burning may emit BC at very
high rates. For example, hard coal combusted in a modern
power plant emits only 2 mg BC per kg of dry matter,
whereas the same fuel burned in residential stoves releases
up to 5200 mg BC per kg, 2600 times more (Bond et al.
2004)! Emission factors for open fires, such as savanna,
grassland, and tropical forest fires, are of intermediate
magnitude, with average values of 600 ± 200 mg BC per
kg of dry matter (Andreae and Merlet 2001). As for diesel
engines, technology can also make a big difference, with
emission factors ranging from 60 mg of BC per kg of fuel
for modern diesel car engines to 4000 mg per kg of fuel
for super-emitters in off-road transport and shipping.
Global emission inventories attribute 20 wt% of annual
BC emissions to biofuel combustion and divide the rest
roughly equally between fossil fuel combustion and open
biomass burning (Bond et al. 2004).
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A ugus t 2010
ATMOSPHERIC BROWN CLOUDS
IN SELECTED REGIONS AND THEIR
CLIMATIC AND ECONOMIC IMPACTS
India and the Northern Indian Ocean
Mean aerosol optical depth (AOD) at visible wavelengths from December 2001 to May 2002 showing
both the spatial extent and intensity of the ABC over the Indian
subcontinent. The data were obtained using the MODIS instrument
onboard NASA’s Terra satellite. AOD = 1 corresponds to the case
when incoming solar irradiation is attenuated to the e −1 fraction
(36.8%) of its top-of-the-atmosphere (almost clear-sky) intensity.
Backscattered radiation intensity can be measured directly by the
satellite, and AOD can be retrieved from this signal using atmospheric radiation models with simplifying assumptions. An AOD of
0.6, as seen in wide areas of India, signifies very high levels of
visible air pollution, levels that are typical during episodes of
massive urban smog. The orange-red areas in Iran, Pakistan, and
Afghanistan represent desert dust. Image courtesy of NASA’s E arth
O bserving System (EOS) project
Figure 3
The atmosphere over this region provides a natural laboratory for studying the effects of air pollution on climate.
Massive air pollution from southern and southeastern Asia
accumulates in the long dry season between November
and May and becomes a haze that spreads over the Arabian
Sea, Bay of Bengal, northern Indian Ocean, and southern
Asia (Fig. 3). According to radiocarbon analyses, biomass
burning produces two-thirds of the carbonaceous aerosol
particles in the region, whereas biofuel and fossil fuel
combustion accounts for one-third (Gustafsson et al. 2009).
Since submicrometer-sized particles are usually removed
by precipitation, the lack of rainfall during the northern
winter results in high levels of visible urban pollution
everywhere in the region (Fig. 3), extending to and sometimes enriched in the free troposphere (Fig. 4). In addition,
these ABCs contain 2–3 times more BC than suburban
aerosol in Europe and North America, due to the much
greater abundance of low-efficiency combustion sources in
the region (see previous section). The combination of these
factors leads to a reduction (by 10–35 W/m2, or 5–15%) of
the incoming solar radiation absorbed by the Earth’s
surface (Fig. 5). Black carbon particles aloft directly absorb
both the incoming solar radiation and the radiation
reflected by the Earth’s surface and low clouds. This lends
a brownish color to the sky and heats the lower atmosphere
by as much as 50–100% while reducing solar radiation at
the surface by about 10% compared to clear-sky conditions
(Ramanathan et al. 2005). Such a significant redistribution
of energy between the Earth’s surface and the atmosphere
reduces evaporation from the ocean, because approximately 50–80% of the radiative heating at the surface is
balanced by evaporation. Moreover, this energy redistribution may shift the monsoonal circulation southwards, as
well as reduce rainfall over land in the region. Because
perturbations by ABCs are not distributed uniformly over
the Earth’s surface, in the dry season, the Indian subcontinent and the northern Indian Ocean receive less energy
due to the dimming effect of ABCs. Furthermore, in highly
polluted air, precipitation is generally suppressed because,
even though more cloud droplets are nucleated, they are
smaller and do not coalesce effectively into raindrops. Lau
et al. (2009) postulated that aerosol-induced, anomalous
mid- and upper-tropospheric warming above the Tibetan
Plateau leads to early onset and a northwestward shift of
Color-coded profiles of a 532 nm backscatter return
lidar signal showing the vertical distribution of ABCs.
The color scale shows aerosol in green, yellow, and red, corresponding to low, medium, and high loadings, respectively. The
topography is shown in dark blue; the lighter blue regime above
the haze layers (yellow and red tones) indicates the free troposphere. Modified from R amanathan et al. (2007)
Figure 4
Seasonal (January to April for the period 1996–1999)
mean reduction in the intensity of solar radiation
absorbed by the Earth’s surface due to the Indo-Asian haze.
Modified from R amanathan et al. (2001)
Figure 5
E lements
226
A ugus t 2010
A
monsoon rainfall—the so-called “elevated heat pump”
effect. Additionally, BC transported over the Himalayas
(Fig. 4) and deposited on snow darkens the surface of snow
fields and glaciers and enhances the absorption of solar
radiation by about 20 W/m2, possibly contributing to the
retreat of the Himalayan glaciers.
B
In addition, economic and societal effects may result from
reduced solar irradiation and rainfall due to the presence
of ABCs over agricultural areas. For instance, a recent
modeling study based on historical records covering a
period of four decades concluded that rice production in
India had been significantly reduced by ABCs through
suppressed precipitation (Auffhammer et al. 2006).
Indonesia
The Indonesian islands and neighboring countries also
experience severe pollution due to ABCs. Agricultural fires
are the main cause of the regional haze: they are common
across the region and occur annually in the dry season
(August to October) despite a legal ban on open burning.
Fires are used frequently as a cheap method of land clearing
for farming or tree planting. These fires often get out of
control and spread into adjacent forests and peat-swamp
areas. Fires in the widespread peat-rich tropical forests,
which are characterized by thick layers of dead, undecayed
vegetation, are particularly difficult to extinguish and
produce exceptionally large amounts of smoke. Some of
the fires may be sustained for weeks or even months until
the monsoon rains start in late autumn. Smoke emissions
from these fires are typically transported in a northwesterly
direction, severely impacting air quality not only in
Indonesia but also in countries situated downwind of the
E lements
(A) Satellite image obtained from the Moderate
Resolution Imaging Spectroradiometer (MODIS) on
NASA’s Terra satellite. Fires are shown in red in Kalimantan, the
Indonesian part of Borneo. Heavy smoke is seen to move northward
over Kalimantan and Sarawak, extending over a distance of more
than 500 km and a few hundred kilometers in width. Image from
Visible E arth, courtesy of NASA’s E arth O bserving System (EOS) project
(B) Aerosol Index (AI) map of southeastern Asia, showing the extensive smoke layer over Indonesia during the severe biomass-burning
season in the autumn of 1997. A positive AI points to the presence
of light-absorbing aerosol particles (negative AI values are related to
purely scattering particles). Image courtesy of NASA’s E arth Probe
TOMS team
227
Figure 6
A ugus t 2010
fires (Singapore, Malaysia, Brunei, Thailand). In 2006, for
instance, millions of hectares of forest and farmland
burned in the dry season, generating a thick layer of smoke
over Indonesia, as seen on satellite images (Fig. 6a). Another
particularly intense burning season occurred in 1997,
when, from September to November, most of Indonesia
was covered by a dense layer of haze (Fig. 6b). The haze
originated from fires on the islands of Sumatra and Borneo,
which devastated 45,600 km 2 of land (Levine 1999).
Astounding peak ambient concentrations of aerosol particles of 4000 µg/m3 (i.e. two orders of magnitude higher
than typical ambient levels in urban areas) were observed
in the fire region during the burning season of 1997 (Heil
and Goldammer 2001). A total of 16.6 Mt of particulate
carbon was emitted during that year, exceeding even the
emissions from the severe 1991 Kuwait oil fires
(Levine 1999).
also contributes substantially to the overall aerosol burden
in developing countries and, thus, constitutes an important
source for atmospheric brown clouds (Yevich and
Logan 2003).
In addition, the frequent use of biomass for residential
heating and cooking in most countries in southern and
southeastern Asia generates substantial amounts of smoke,
as a large fraction of the population lives in rural areas in
this part of the world. Aside from severe indoor air pollution caused by such practices, domestic biofuel combustion
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E lements
CONCLUDING REMARKS
As the human population associated with expanding
urbanization increases, we can expect that ABCs will
become more intensive in the future, impacting nearly half
of the world’s population. ABCs show us how the unwanted
effects of small-scale human activities merge into spectacular phenomena that are visible from space and exert
significant supraregional forcing in the Earth’s atmosphere.
Given the complexity of their effects over vast scales, ABCs
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